A hub unit bearing is widely used to rotatably support a vehicle wheel of an automobile and a rotating member for braking such as a brake rotor to a suspension device. For example, in a related-art hub unit bearing described in JP-A-2009-149142, a hub which is a rotating side raceway ring is rotatably supported by a plurality of rolling elements on an inner diameter side of an outer ring which is a stationary side raceway ring. The hub includes a rotating side flange having an outward flange shape for supporting and fixing a vehicle wheel on an outer peripheral surface thereof.
Referring to FIGS. 5A and 5B, a related-art rotating side flange 50 is formed with a bolt hole 54 axially penetrating the rotating side flange 50. A wheel configuring a vehicle wheel (not shown) and a brake rotor configuring a braking device are supported and fixed by a hub bolt 57 fixed to the bolt hole 54. The hub bolt 57 is press-fitted and fixed with the rotating side flange 50 by serration-fitting into the bolt hole 54 having the serration portion 58. A radially outer portion of the bolt hole 54 has a stepped shape where a large-diameter portion 51 having a large outer diameter dimension is connected to a small-diameter portion 53 having a small outer diameter dimension by a step portion 52. Further, a mounting side chamfer 55 and a counter-mounting side chamfer 56 are formed on both opening portions of the bolt hole 54, respectively. The mounting side chamfer 55 on a side mounted with the brake rotor is corner chamfered at two stages, so that the flange vibration accuracy of the rotation side flange 50 is improved and the occurrence of brake judder is reduced.
In the above related-art hub unit bearing, it is difficult to ensure the form deviation and the dimensional accuracy of the bolt hole 54 formed in the rotating side flange 50 by drilling.
That is, a radial thickness dimension (thickness) of a portion of the bolt hole 54 on an outer diameter side becomes thick at a portion radially overlapping with the large-diameter portion 51 and becomes thin at a portion radially overlapping with the small-diameter portion 53 with the step portion 52 as a boundary. Therefore, the rigidity around the bolt hole 54 also increases at the large-diameter portion 51 and decreases at the small-diameter portion 53. Therefore, the bolt hole 54 may be bent radially outward from the vicinity of the step portion 52, and an inner diameter dimension of the bolt hole 54 may change from the vicinity of the step portion 52 (the large-diameter portion 51 has a larger inner diameter dimension than the small-diameter portion 53). The fitting portion X1 where the serration portion 58 of the hub bolt 57 is serration-fitted to the bolt hole 54 extends over the step portion 52 and radially overlaps with both the large-diameter portion 51 and the small-diameter portion 53.
When the bolt hole 54 is bent in a middle, the hub bolt 57 press-fitted (serration-fitted) to the bolt hole 54 may be inclined. Therefore, the workability during the operation of mounting the wheel configuring the vehicle wheel and the rotating member for braking may be reduced. Further, when a tightening state of a nut is controlled by a level of a tightening torque, a variation in an axial force corresponding to the level of the tightening torque may increase and unnecessary stress may tend to act on the bolt.
When the inner diameter dimension of the bolt hole 54 changes in a middle, a fitted state of the serration portion 58 formed on an outer peripheral surface of the hub bolt 57 and an inner peripheral surface of the bolt hole 54 becomes loose at the large-diameter portion 51 and becomes strong at the small-diameter portion 53. Therefore, a fitting force may become unstable at the fitting portion.